Our studies have focused on a TPS system that was previously described in the annotation of the genome of E. coli O157:H7 strain EDL933. We designated the exoprotein and transporter components of this system OtpA and OtpB, respectively. We found that OtpA and OtpB are not predicted to belong to either of the two major subtypes of TPS systems (hemolysins and adhesins) based on their sequence. Nevertheless, we obtained direct evidence that OtpA and OtpB constitute a bona fide two-partner secretion system. We found that secretion of OtpA into the extracellular environment in E. coli O157:H7 requires OtpB and that when OtpA is produced in an E. coli K-12 strain, its secretion is strictly dependent on the production of OtpB. Furthermore, we have used OtpA/OtpB as a model system to characterize the kinetics of secretion via the TPS pathway. Despite the fact that the exoprotein must be targeted to the transporter before it is extruded into the extracellular space, we found that protein secretion via the TPS pathway is extremely rapid. Recently, we have been using the OtpA/OtpB system to gain insight into the mechanism of exoprotein secretion. Most notably, we have obtained strong evidence that OtpA is transported across the inner and outer membranes in two sequential steps.